Until recently, mobile wireless equipment used separate Integrated Circuits (ICs) for the Radio Frequency (RF) transceiver, the baseband (BB) processor and the Power Management Unit (PMU). With the growing success of FDD-WCDMA (3G) telecommunication standards, HSPA (acronym which covers HSDPA and HSUPA application) as part of FDD-WCDMA technology, which used to be a premium telecommunication feature targeting only high-end phones, must now be incorporated with limited extra cost to the previous 2.5 G generation of handsets, such as EGPRS devices. In this context, the approach taken by most IC vendors is that of a single chip, made up of either a single RF CMOS die, or multiple separate dies, which integrates into a single package all three previously listed ICs, namely, RF, BB and PMU into a single package.
FIG. 1 illustrates the general architecture of a multiple-band 2G/3G phone 100 which shows such an integration of a RF Front End circuit 110, a 2G/3G RF transceiver 120, a baseband 130, a PMU unit 140 and possibly DDR memory 150, being either external or internal.
FIG. 1 shows that RF Front-End circuit 110 supports quad band 2G (Band II, III, V, VIII EGPRS), triple band 3G (WCDMA I, II, III) which is typical of recent mobile phone architecture, the selection of the particular mode/band being performed by means of an antenna switch 111 which directs the signal to the appropriate set of front end filters 112. Conversely, antenna switch 101 directs the transmit signal generated by the appropriate 2G or 3G Power amplifiers, respectively 113 and 114, to the antenna.
2/3G transceiver 120 includes the conventional circuits required for achieving a 2G or 3G mobile communication, such as, in the receiving chain, low Noise amplifiers (LNA) 121, a Rx VCO Frequency synthesizer 122 with appropriate division circuits (represented by local divider LO Div), a circuit 123 achieving programmable Gain amplifier (PGA), Analog to digital converter (ADC) as well as DSP processing. On the transmitting chain, transceiver 120 includes a circuit 126 achieving PGA, Digital to Analog (OAC) conversion as well as DSP processing, a Tx VCO frequency synthesizer 125 associated with dividing circuits (LO Div), and conventional digitally controlled Gain amplifier 124. Transceiver 120 further includes appropriate timing circuits 126 as well as a RF-BB baseband interface 127 for interfacing the baseband 130. For the sake of clarity, the different control, data and clock signals which are represented in FIG. 1 (such as RFBBi_EN, RX data 1, RX data 2, TX data 1, SYSCLKEN, SYSCLK) are conventional and known to the skilled man and do not need any further discussion.
Similarly, baseband 130 achieves communication between the transceiver 120 (through interface 127) with different devices and peripherals, such as two cameras 160, two displays 170, a USB device 180 through appropriate data and control leads (including CLK clocks and Chip Select CSi) as well as external DDR memory.
It can be seen that the integration of those components in a single die clearly reduces the cost of manufacturing a handset since the telecom pipe of the mobile phone now only requires very few extra additional components to make a phone call: one or several Power Amplifier(s) (PA) and its associated front-end circuitry such as RF bandpass filters, duplexers, antenna switch etc.
While the single chip RF, BB, PMU presents a significant cost reduction of the entire mobile phone chipset, there are significant EMI problems to be considered in order to prevent the RF receiver as well as the RF transmitter chain from being polluted by digital BB and external memory bus noise, as well as associated clock spurs, and their multiple harmonics.
In such a context, the problem of the EMI interference introduced in the 3G receiver chain as being the victim shows to be highly critical.
Indeed, the 3G receiver suffers from multiple sources of aggression which can be sorted into two families:                Wideband noise source aggressors: falls into this category, noise generated by high speed data transfers between the single chip and its peripherals, such as camera 160, displays 170 but also, USB 180 and external memory interface 150,        Narrowband spurs: falls into this category, either clock harmonics spurs, or pulsed clock source and their harmonics.        
Aggressor's basic spectral properties are illustrated in FIGS. 2a-2c with respect to three typical situations:
FIG. 2a: continuous clock harmonics,
FIG. 2b: NRZ or RZ continuous data transmission,
FIG. 2c: pulsed clock source.
It should be noticed that since the victim is a 5 MHz wide carrier, it is not excluded that several spurs might fall within the victim's receiver carrier bandwidth.
The digital circuitry generates narrowband spurs which span over several hundred of MHz, which can couple into the Low Noise Amplifier (LNA) input pin(s) via electromagnetic coupling of the long bonding wires which can be modeled as radiating transmissions lines, acting in a fashion very similar to antennas.
FIG. 3 below shows the impact of the presence of a single Continuous Wave (CW) tone in-band jammer onto the receiver sensitivity of a typical 3G victim (NF=6.4 dB). There is plotted the NF User Equipment (UE) loss of receiver sensitivity vs. DPCH_Ec to Jammer power ratio (DPCH_Ec/J) for different UMTS cell geometry factors (Ior/Ioc power ratios). Fw=wanted signal carrier frequency, fj=jammer carrier frequency. The Minimum 3GPP conformance test requirements are shown with a vertical bar.
It should be noticed that such analysis is also applicable to the case of a single GMSK modulated carrier falling inside the receiver bandwidth (BW). For the sake of clarity the frequency offset dependent tolerance of the victim to the presence of such a jammer has not been represented in FIG. 3. Note that the geometry factor curve of −6 dB has been plotted here only for illustration purposes since the problem addressed by the present invention is impacting the receiver only when the downlink received modulated carrier power (Ior) is close to the UE reference sensitivity level. This situation corresponds to a UE located at cell edge. In conformance tests, the applicable set of curve is that where no additional AWGN noise is used (curve with Ior/Ioc=infinity).
The set of curves plotted in FIG. 3 shows how critical might be the introduction of additional clock spurs resulting from a digital activity in the most sensitive parts of the analog circuitry, and particularly when Ior gets close, or below the 3GPP conformance required sensitivity level.
Some prior art solutions are already known for limiting the effects of such additional spurs.
A first solution consists in protecting the victim by carefully designing the LNA and systematically using differential wires in order to make best benefit of the common mode rejection of such a differential architectures.
A second known solution is based on the use of sophisticated packages (eg. so-called flip chip package) for embodying the RF transceiver integrated circuit may reduce the coupling between the input wire of the LNA and the digital interface, which coupling generally increases with the frequency.
All those techniques clearly tend to increase the design and manufacturing costs of the transceiver IC.
In some situations, those techniques do not allow to avoid desensitization of the receiver in some circumstances. In particular, it has been shown that multiple integer harmonics of the reference clock used to transfer data over the external memory interface desensitize and/or degrade an RF receiver.
This is particularly shown in FIG. 4 for a 2G sensitivity measurement performed across all operating channels of the 900 MHz band, with an example of a reference clock of 26 MHz.
It can be clearly seen in FIG. 4 that severe reference sensitivity occurs at carrier frequency 936 MHz. This problem results from the fact that 936 MHz is a carrier frequency which is equal to the 36th multiple integer of the reference clock rate used by both DSP/CPU and external memory access. Therefore the 36th harmonic couples into the LNA input pins, and therefore degrades the 2G sensitivity.
The observations above show how critical the problem of EMI interference can be in a 3G receiver.
Therefore, there is a need of an effective technique allowing to protect the 3G receiver from the EMI pollution introduced by aggressors which are integrated in the same IC circuit.
Furthermore, there is a need for a technique also addresses the problem of the presence of such spurs in the case of a 3G receiver.